This invention pertains to optical fiber Raman amplifiers and to optical fiber communication systems comprising such amplifiers and more specifically to a system and method for pumping the transmission fiber of an optical fiber telecommunication span to produce distributed Raman gain in the fiber for amplifying the signal(s) being transmitted along the fiber span.
Until relatively recently, the amplification of optical signals in fiber-optic telecommunication systems has been achieved primarily through the use of discrete optical amplifiers, mainly erbium-doped fiber amplifiers (EDFAs). The explosive growth in the demand for increased capacity in fiber-optic communication systems has resulted in renewed interest in using distributed Raman amplification. See for instance, P. B. Hansen et al., IEEE Photonics Technology Letters, Vol. 9 (2), p. 262, (February 1997). In this approach, the transmission fiber itself is used as an amplifying medium for signals as they travel towards a repeater or receiving terminal, and the resulting gain is distributed over a length (typically tens of kilometers) of the fiber. Distributed amplification has an important advantage over discrete amplification. The effective noise figure of a distributed amplifier is significantly lower than that of a discrete amplifier having the same gain. See for instance, P. B. Hansen et al., Optical Fiber Technology, Vol. 3, p.221, (1997). This is a direct result of the gain occurring back in the span rather than at the end. The resulting improvement in noise performance not only allows increased capacity and/or span length in unrepeatered systems, but also allows for an increase in the number of spans between costly signal regenerators in multi-span repeatered systems. In addition, Raman amplification offers the possibility of ultra-broadband amplification, since the Raman gain spectrum in silica fiber, even for a single pump wavelength, is relatively broad and can be broadened further by using multiple pump wavelengths. See for instance, K. Rottwitt et al., Proceedings Optical Fiber Communication Conference, Paper PD-6, (February 1998). This is an important consideration for high-capacity wavelength division multiplexed (WDM) systems.
To produce Raman gain in the transmission fiber for signals in a particular wavelength band requires that the fiber be pumped at a relatively high-power level (hundreds of mW) at a wavelength, or wavelengths, shifted down from the signal wavelength(s) by an amount corresponding to the characteristic Raman shift of the fiber. For typical silica fiber, the Raman gain spectrum consists of a relatively broad band centered at a shift of xcx9c440 cmxe2x88x921. Therefore, to provide gain for signals in the C-band (1530 to 1565 nm) for example, requires pump energy in the 1455-nm region.
In typical prior-art distributed Raman amplification embodiments, the output of a high-power laser source (e.g. a Raman fiber laser with a center wavelength of xcx9c1455 nm) or a group of multiplexed laser diodes with wavelengths in the 1455-nm region is launched from a receiving or repeater terminal to pump the fiber and provide gain for the incoming C-band signals. To extend the amplification bandwidth for high-capacity WDM systems, the launched pump spectrum is broadened by using multiple Raman lasers (each with a predetermined power and wavelength) or by multiplexing additional laser diodes of specific wavelength and power.
A characteristic set of power vs. distance curves for the pump, the signals and the noise generated by the amplification process are shown in FIG. 1 (in this graph, distance is referenced from the receiving or repeater terminal). As can be seen in FIG. 1, the gain region is distributed over a length of the transmission fiber extending xcx9c70 km back into the span. However, the bulk of the gain occurs in the last xcx9c15 km of the span. To further increase the noise performance advantage of distributed Raman amplification, it is desirable to pump the transmission fiber in a manner which xe2x80x9cpushesxe2x80x9d the gain region further back in the span.
In K. Rottwitt et al., Proceedings European Conference Optical Communication, Vol. II, p.144, (September 1999), the authors report a pumping scheme which involves launching a high-power (800 mW) source at a wavelength of 1366 nm from the transmitter terminal, to provide Raman gain along the transmission fiber for 1455-nm energy launched from the receiving terminal. Thus, the power at 1455 nm, which provides the Raman gain for the signals at 1550 nm, is amplified along the fiber according to the local value of the power at 1366 nm. As a result, for the particular case they considered (an 80-km long span with 200 mW at 1455 nm launched from the receiving terminal), a substantial amount of signal gain occurs near both the transmitter and receiver ends of the span and the gain, on average, occurs further back in the span. The authors measured a 3-dB improvement in noise figure and a 1-dB improvement in receiver sensitivity (or link margin) as compared to conventional backward pumping. However, this pumping scheme, particularly if it is to be applied to longer spans (e.g. 125 km), requires two relatively expensive, relatively high-power (in the many hundreds of mW) sources. This disadvantage is exacerbated for the case of high-capacity WDM systems where broadening the gain bandwidth would require even more such sources. In addition, in links where the launch power of the signals is at or very near the limit set by considerations of adverse nonlinear effects in the fiber, the addition of substantial amplification immediately after signal launch could lead to link performance impairments due to these nonlinear effects.
Despite the already-demonstrated potential of distributed Raman amplification for providing low-noise, broadband amplification, there is an ever-present need for further performance improvements and cost reductions in optical communication systems. Thus, a distributed Raman amplifier pumping scheme, such as that disclosed in this application, which results in still lower noise and an increased flexibility and cost effectiveness in broadening and dynamically controlling the gain spectrum is highly desirable.
In a broad aspect, the invention provides a pumping scheme for producing distributed Raman amplification in the transmission fiber of an optical fiber communication system, according to which, the high pump power at the wavelength(s) required for amplification of the transmitted signals is developed within the transmission fiber itself, rather than being launched directly into the fiber. This pumping method can result in significantly lower amplifier noise and increased flexibility and cost effectiveness in broadening and dynamically controlling the gain spectrum compared to prior-art pumping methods.
More specifically, in a typical exemplary embodiment, a xe2x80x98primaryxe2x80x99 pump source at a predetermined wavelength xcexp, shorter than the ultimately desired pump wavelength(s) xcexf . . . xcexfk, is launched into the transmission fiber along with one or more lower-power, low-cost secondary xe2x80x98seedxe2x80x99 sources at wavelength(s) xcexs1 . . . xcexsn, where nxe2x89xa71 and xcexp less than xcexsnxe2x89xa6xcexfk. The wavelength and power of the secondary seed source(s) are specifically chosen such that, in the presence of the pump power at xcexp, a series of n stimulated Raman conversions ultimately lead to high power at the final desired pump wavelength(s) xcexf1 . . . xcexfk, where kxe2x89xa6n, being present in the transmission fiber.
In a particular exemplary embodiment, a primary pump source at a wavelength of 1276 nm is launched together with two lower-power secondary sources having wavelengths of 1355 and 1455 nm. Energy at the primary pump wavelength of 1276 nm first undergoes a stimulated Raman conversion to 1355 nm and then, in the second step of a Raman cascade, the resulting high power at 1355 nm is converted to yield high power at 1455 nm, the pump wavelength required to produce distributed Raman amplification of signals in the 1550-nm region. In an extension of this exemplary embodiment, a third low-power source having a wavelength of xcx9c1430 nm is launched along with the above mentioned primary and secondary sources. As a result of the presence of this additional seed source, the conversion of the high power at 1355 nm in the second step of the Raman cascade is shared between the two wavelengths, 1430 and 1455 nm, leading to high power at both these wavelengths, and a broadening of the resulting Raman gain profile for the transmitted signals. In accordance with the invention, further broadening of the gain spectrum can be achieved by launching additional low-cost seed sources of predetermined wavelength and power. Furthermore, the magnitude and the spectral the profile of the gain can be dynamically controlled by selectively altering the power of the low-power seed sources. Additional flexibility in the dynamic control of the gain spectrum results if the wavelength of one or more of the secondary seed sources is tunable.
In another exemplary embodiment, which is a variant of the foregoing example, the secondary source at 1355 nm is replaced by a reflection means (e.g. a gold reflector or a fiber Bragg grating with peak reflectivity at 1355 nm). Spontaneous Raman scattering of the high-power primary pump at 1276 nm produces radiation in the 1355-nm region traveling in both directions in the transmission fiber. As it travels in the fiber, this 1355-nm radiation is amplified due to the Raman gain at 1355 nm produced by the 1276-nm pump. In addition, some of the outgoing 1355-nm radiation undergoes Rayleigh backscattering and heads back towards the pump launch terminal, being further amplified as it goes. The 1355-nm reflector sends the incoming amplified spontaneous Raman scattered radiation back into the transmission fiber, where it performs the same role as the 1355-nm seed source in the foregoing example. In an extension of this exemplary embodiment, the 1455-nm seed source is also replaced by a reflector, thereby totally eliminating the need for active secondary sources, such as laser diodes, and further reducing costs.
In preferred embodiments, the primary pump and secondary seed source(s) are counter propagating with respect to the transmitted signals. However, the invention also provides for co-propagating configurations which can be used to advantage compared to prior-art co-pumping methods, since the peak gain region occurs at some distance from the transmitter terminal, thereby lessening adverse non-linear effects.
In a further embodiment, with counter-propagating distributed Raman preamplification being applied at the receiving end of a span, one or more moderate-power sources at wavelengths xcexss1 . . . xcexssj, where the xcexssj are shorter than the xcexfk by an amount corresponding to the Raman shift in the transmission fiber, are launched from the transmitter terminal, thereby providing Raman gain for the residual incoming counter-propagating final pump wavelengths xcexfk, which in turn provides some distributed Raman amplification for the outgoing signals. The magnitude and/ or spectral profile of this additional Raman amplification can be dynamically controlled by selectively altering the power and/or wavelength (if any xcexssj is tunable) of the moderate-power source(s), thereby allowing dynamic gain control and flattening to be applied near the transmitter terminal rather than at the receiving terminal, where it is less effective due to time-of-flight issues. This application of the invention can also be used to advantage in cases where the traditional direct Raman pumping scheme is being utilized at the receiving terminal.
According to a broad aspect of the present invention, there is provided a method of pumping the transmission fiber of an optical fiber telecommunications span to produce distributed Raman gain in the fiber for signals being transmitted along the fiber span, the method comprising the steps of: providing one or more primary pump sources at wavelengths xcexp1 . . . xcexpi, shorter than the pump wavelengths xcexf1 . . . xcexfk ultimately required to directly produce distributed Raman gain for the signal wavelengths; providing substantially lower energy at one or more secondary seed wavelengths xcexs1 . . . xcexsn, where nxe2x89xa71 and xcexpi less than xcexsnxe2x89xa6xcexfk; and propagating the energy at the primary pump wavelengths and secondary seed wavelengths in the transmission fiber; and wherein the primary pump wavelengths xcexpi are less than the wavelengths xcexfk by an amount corresponding to m Raman shifts in the transmission fiber, where mxe2x89xa71, and where, if m greater than 1, the ensemble of secondary seed wavelengths xcexsn includes at least one in the vicinity of each intermediate wavelength xcexl, where l=mxe2x88x921,mxe2x88x922 . . . 1, and denotes the number of Raman shifts in the transmission fiber between the wavelength xcexl and the ultimately required wavelengths xcexfk; and wherein the ensemble of the secondary seed wavelengths xcexsn includes each ultimately required pump wavelength xcexfk.
According to another broad aspect of the present invention, there is provided a method for applying dynamic control of the magnitude and/or the spectral profile of the distributed Raman gain at, or near, the signal launch terminal of an optical fiber telecommunications span in which counter-propagating distributed Raman preamplification is being applied at the receiving or repeater end of the span or at some intermediate point along the span, resulting in residual energy at the final direct-pumping wavelengths xcexfk nearing the signal launch terminal, the method comprising the steps of: providing one or more moderate-power secondary pump sources at wavelengths xcexss1 . . . xcexssj, shorter, by an amount corresponding to the Raman shift in the transmission fiber, than the pump wavelengths xcexf1 . . . xcexfk ultimately required to directly produce distributed Raman gain for the signal wavelengths; providing coupling means to input radiation from said secondary pump sources at the xcexssj into the transmission fiber from the signal launch terminal of the span or from an intermediate point near said signal launch terminal, to travel in a co-propagating direction with respect to the signals; and providing means to selectively alter the power and/or the wavelength of said secondary pump sources at the xcexssj to dynamically control the Raman gain experienced by the incoming radiation at the final direct-pumping wavelengths xcexfk, and thereby to dynamically control the magnitude and/or the spectral profile of the resulting additional Raman gain experienced by the signals being launched.
According to a further broad aspect of the present invention, there is provided a system for pumping the transmission fiber of an optical fiber telecommunications span to produce distributed Raman gain in the fiber for amplifying signals being transmitted along the fiber span, and which comprises: one or more primary pump sources at wavelengths xcexp1 . . . xcexpi, shorter than the pump wavelengths xcexf1 . . . xcexfk ultimately required to directly produce distributed Raman gain for the signal wavelengths, wherein the one or more primary pump source wavelengths xcexp1 . . . xcexpi are shorter than the wavelengths xcexfk by an amount corresponding to m Raman shifts in the transmission fiber, where mxe2x89xa71; means to provide substantially lower energy at one or more secondary seed wavelengths xcexs1 . . . xcexsn, where nxe2x89xa71 and xcexpi less than xcexsnxe2x89xa6xcexfk, and where, if m greater than 1, the ensemble of the one or more secondary seed wavelengths includes at least one in the vicinity of each intermediate wavelength xcexl, where l=mxe2x88x921, mxe2x88x922 . . . 1 and denotes the number of Raman shifts in the transmission fiber between the wavelength xcexl, and the ultimately required wavelengths xcexfk, and wherein the ensemble of the one or more secondary seed wavelengths xcexsn includes each ultimately required pump wavelength xcexfk; and coupling means to input energy from the one or more primary pump sources at wavelengths xcexp1 . . . xcexpi and energy at the one or more secondary seed wavelengths xcexs1 . . . xcexsn, into said transmission fiber.
According to another broad aspect of the invention, there is provided a system for applying dynamic control of the magnitude and/or spectral profile of the distributed Raman gain at, or near, the signal launch terminal of an optical fiber telecommunications span in which counter-propagating distributed Raman preamplification is being applied at the receiving end of the span, which comprises: one or more moderate-power secondary pump sources at wavelengths xcexss1 . . . xcexssj, shorter than the pump wavelengths xcexf1 . . . xcexfk ultimately required to directly produce distributed Raman gain for the signal wavelengths, wherein the one or more secondary pump source wavelengths xcexss1 . . . xcexssj are shorter than the wavelengths xcexfk by an amount corresponding to the Raman shift in the transmission fiber; residual radiation at the final direct-pumping wavelengths xcexfk travelling toward the signal launch terminal in a counter-propagating direction with respect to the signals; coupling means to input radiation from the one or more secondary pump sources at wavelengths xcexss1 . . . xcexssj into the transmission fiber from the signal launch terminal of the fiber span, or from an intermediate point near the signal launch terminal, to travel in a co-propagating direction with respect to the signals; and means to selectively alter the power and/or wavelength of the one or more secondary pump sources at wavelengths xcexss1 . . . xcexssj to dynamically control the Raman gain experienced by the incoming radiation at the final direct-pumping wavelengths D and thereby to dynamically control the magnitude and/or spectral profile of the resulting additional Raman gain experienced by the signals being launched.